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A magnetic chimney in NGC 6946

Submitter: George Heald
Description: The interstellar medium (ISM) in galaxies is directly a ffected by outflows caused by star formation. Magnetic fields are an important ingredient of the ISM, but we don't know much about their connection to the gas and its evolution. Recently, some progress has been made through a novel combination of two WSRT datasets. Today's image shows the Faraday rotation measure (RM) across the disk of the spiral galaxy NGC 6946 - red means a magnetic field pointing toward you, and blue indicates a field that is pointing away. The inset at the lower left shows a zoom-in of the newly discovered feature. The black circle indicates the boundary of a huge hole in the neutral hydrogen (HI) distribution, which itself was catalogued by Boomsma et al. in 2008.

The combined characteristics of this newly discovered feature point to a picture like the so-called "chimney model" (described by Norman & Ikeuchi in 1989). The cartoon in the lower right is copied from their paper, and it shows the main features of this model. Hot gas is pushed out of the galaxy disk by multiple supernovae, leaving a gaping hole in the HI distribution. The hot gas carries the magnetic field along for the ride, leading to a vertical displacement of the initially flat magnetic field. After a few tens of millions of years, the gas cools and returns to the disk (this is also observed). But the magnetic field is left behind, sticking up into the halo of the galaxy. Its effect on the RMs is to leave the red-blue pattern that you see inside the HI hole.

This is the first time that magnetic fields have been directly observed to be participating in such a disk-halo interaction, but we think that there may be many more of these "magnetic chimneys" hiding in galaxies like NGC 6946. They are very important both for the evolution of the ISM in galaxies, as well as for the development of magnetic fields. You can find more details here.
Copyright: ASTRON
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